1. Field of the Invention
The present invention relates to a direct-current power supply device and particularly to a technique of reducing a rise in voltage of a smoothing capacitor of a PFC (Power Factor Correction: Power factor improvement) circuit at a time when a load is light in a direct-current power supply device designed to control a DC/DC converter and the PFC circuit by means of one switching element.
2. Description of the Related Art
A direct-current power supply device, which converts a commercial alternating-current power supply to a direct-current power supply using a rectification smoothing circuit and then converts the direct-current power supply to a desired direct-current voltage using a DC/DC converter to output the direct-current voltage, has been used. When the direct-current power supply is obtained by the rectification smoothing circuit from the commercial alternating-current power supply, current flows through a smoothing capacitor only around a peak of sine-wave alternating-current voltage. Accordingly, a power factor becomes worse; a higher harmonic wave is generated, which badly affects surrounding areas. To solve the above problem, a PFC circuit may be provided in the rectification smoothing circuit. In this case, a switch used by the PFC circuit can be shared as a switching element by the DC/DC converter; the sharing of the switch is effective in making the direct-current power supply device smaller and reducing costs. Those sharing the switching element between the PFC circuit and the DC/DC converter include, for example, the one disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2002-247843 (Patent Document 1) or U.S. Pat. No. 5,991,172 (Patent Document 2).